Things go in cycles. Have you noticed? Second Life™ tutorials certainly seem to come in waves. For instance Strawberry Singh has redone her Avatar Physics tutorial as a video. See: Second Life Avatar Physics Tutorial or jump directly to the video tutorial at YouTube. I suggest you read Strawberry’s article.
Strawberry’s is a nine minute video that covers the information you can find detailed in the Firestorm Viewer’s wiki: Firestorm Avatar Physics, with a bit more detail. Everything you really need to know is in the video. I think she did a pretty good job on the video.
She points out that your mesh body and mesh clothes have to be rigged for Avatar Physics, otherwise your boobies and other parts won’t bounce. The only way you can know if an item is correctly rigged is to try the demo. Merchants just don’t put this stuff in the promo material, at least not in-world. They are a bit better in the marketplace.
According to what Strawberry is told only the Belleza body has butt bounce. From firsthand experience she says Slink and Maitreya bodies do not, just breast and belly bounce.